1. Field of the Invention
The present invention relates to nano-sized melanin particles having excellent dispersibility in a solvent, a preparation method thereof, and an intermediate for the preparation of melanin particles.
2. Description of the Related Art
Melanins are biopolymers that are widely distributed in many parts of living organisms such as plants, animals, and protista, and are usually categorized into black-brown eumelanins and yellow-reddish pheomelanins. Eumelanins are derived from 3,4-dihydroxy-L-phenyl alanine (L-DOPA) or 2-(3,4-dihydroxyphenyl)ethylamine (dopamine), and pheomelanins are derived from L-DOPA or dopamine in the presence of thiol group (—SH)-containing compounds such as cysteine and glutathione. Eumelanins are predominantly found in mammals, and are known to be biopolymers having irregular polymeric structures, including the indole units which are formed from catecholamines by intramolecular addition of the amino groups to the oxidatively generated o-quinones.
Many studies have been actively conducted melanins and their methods of synthesis and applications thereof because of their various biological functions as well as the function of blocking UV radiation as a pigment.
Melanins have been reported to have a diverse number of biological functions, including photoprotection by absorbing a broad range of electromagnetic radiation, photosensitization, metal ion chelation, antibiotic, thermoregulation, and free radical quenching. Melanins are widely used in various fields such as photovoltaic cells, sensors, optoelectric and energy storage, photoactive and photoprotective materials, antioxidant materials, biomedical applications, and cosmetics.
Melanins can be obtained from natural sources or by artificial synthetic methods using enzymes or oxidants.
Specifically, melanins can be obtained in vivo from oxidation of an amino acid, tyrosine, by tyrosinase into dihydroxy phenylalanine (DOPA), followed by a series of oxidation reactions of dihydroxy phenylalanine. However, melanins thus obtained from a natural source are insoluble in a solvent such as water, which is problematic when the melanins are applied in various fields. In addition, the production amount is not constant, and there is no accepted set of standardized procedures that has been proven to produce natural melanins in their pure forms. Artificially synthesized melanins have a latent limitation in their practical application because of their irregular morphologies, the long time required for synthesis, and poor solubility and dispersibility in a solvent such as water.
Synthetic melanin methods usually cannot provide nano-sized particles that are dispersible in a solvent such as water. Also, natural melanins are not dispersed in solvents.
The present inventors polymerized melanins by oxidation of dopamine resulting from the chemical reaction of a dopamine.H+X−-containing aqueous solution (wherein H+X− is an acid) with a base. They found that upon addition of the base, a molar ratio of dopamine.H+X− and the base is controlled to prepare melanins with a particle shape, and furthermore to control the particle size in a nanoscale. The present invention has been completed on the basis of this finding.
Further, the present invention provides melanin particle prepared by polymerization of DOPA (3,4-dihydroxyphenylalanine) using an oxidizing agent having a standard reduction potential of approximately 1.4-2.
Furthermore, the present invention is intended to precisely control the size and shape of melanin particles by controlling several synthetic conditions influencing the reaction rate.